[英]Covariance Generics c#
This is an example taken from MSDN , for covariance in generics in C#. 这是来自MSDN的示例,用于C#中泛型的协方差。
I am unable to print FullName, can I know why the output is not printing? 我无法打印FullName,我可以知道为什么不打印输出吗?
// Simple hierarchy of classes.
public class Person
{
public string FirstName { get; set; }
public string LastName { get; set; }
}
public class Employee : Person { }
public class Print
{
// The method has a parameter of the IEnumerable<Person> type.
public static void PrintFullName(IEnumerable<Person> persons)
{
foreach (Person person in persons)
{
Console.WriteLine("Name: {0} {1}",
person.FirstName, person.LastName);
}
}
}
class Program
{
public static void Main()
{
IEnumerable<Person> employees = new List<Person>();
Person person = new Person();
person.FirstName = "nuli";
person.LastName = "swathi";
Print.PrintFullName(employees);
Console.ReadKey();
}
}
Because your employees
list is empty. 因为您的
employees
列表为空。
You should add your Person instance to the employees
list, then you'll see it printed. 您应该将Person实例添加到
employees
列表,然后将其打印出来。
ie 即
class Program
{
public static void Main()
{
IList<Person> employees = new List<Person>(); //you need to declare this as a List/IList to get the `Add` method
Person person = new Person();
person.FirstName = "nuli";
person.LastName = "swathi";
//add this line
employees.Add(person);
Print.PrintFullName(employees);
Console.ReadKey();
}
}
Is it possible you forgot the string.format? 您是否有可能忘记了string.format? Try to use this:
尝试使用此:
Console.WriteLine( String.format("Name: {0} {1}", person.FirstName, person.LastName))
instead of this: 代替这个:
Console.WriteLine("Name: {0} {1}", person.FirstName, person.LastName);
or even better this: 甚至更好:
Console.WriteLine($"Name: {person.FirstName} {person.LastName}");
ref: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/tokens/interpolated 参考: https : //docs.microsoft.com/zh-cn/dotnet/csharp/language-reference/tokens/interpolated
EDIT: Since its a mdn example its more likely the list is empty.. 编辑:由于其mdn示例其更有可能的列表为空。
Just use a List, List implements IEnumerable. 仅使用List,List实现IEnumerable。 There were a few mistakes within your Program class, You can find the corrected code below:
您的Program类中存在一些错误,您可以在下面找到更正的代码:
public class Person
{
public string FirstName { get; set; }
public string LastName { get; set; }
}
public class Employee : Person { }
class Print
{
// The method has a parameter of the IEnumerable<Person> type.
public static void PrintFullName(IEnumerable<Person> persons)
{
foreach (Person person in persons) {
Console.WriteLine(string.Format("Name: {0} {1}", person.FirstName, person.LastName)); // needs to format the string for it to work, hence string.format().....
}
}
}
public class Program
{
public static void Main()
{
List<Person> employees = new List<Person>(); // no need to convert it to an IEnumerable object as List already implements IEnumerable<>
Person person = new Person();
person.FirstName = "nuli";
person.LastName = "swathi";
employees.Add(person); // You have to populate the list first
Print.PrintFullName(employees);
Console.ReadLine(); // Changed from Console.ReadKey()
}
}
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